Models insert() and save() methods return "unknown error" but the query builder works

I'm trying to get user creation functionality working on a signup page. When the form is submitted I use either

Code:

$user->insert($data)

or

Code:

$user->save($data)

. When I call

Code:

$user->error()

it returns "Unknown error" with a code of "101".

If I try to insert the same date using the Query Builder then it works correctly.

Am I using the model class incorrectly?

Here is my code for the method:

PHP Code:

public function create()
 {
        $user = new User();

        $rules = [
            'username' => 'required|min_length[2]|alpha_numeric',
            'email' => 'required|valid_email',
            'password' => 'required',
            'password_confirm' => 'required|matches[password]'
        ];

        /**------------------------------------------------------------------------------
        * Validation
        ------------------------------------------------------------------------------*/
        if (! $this->validate($user->validationRules))
        {
            echo view('login/signup', [
                'validation' => $this->validation,
            ]);
        } else
        {
            $username = $this->request->getVar('username');
            $email = $this->request->getVar('email');
            $password = $this->request->getVar('password');

            $userFound = $user->where('username', $username)->first();
            $emailFound = $user->where('email', $email)->first();

            $data = [
                'username' => $username,
                'email' => $email,
                'password' => password_hash($password, PASSWORD_DEFAULT)
            ];

            dump($user->insert($data));
            dump($user->error());

            $this->session->setFlashdata("message", "An account for <strong>{$username}</strong> has been created. Now you can login!");
            return redirect()->to('/login');
        }